DISCLAIMER: This project plugs in to the mains, which has dangerous voltages and can kill you. Never plug in the device working on this project – you can re-program using an external 5V supply wired up to the same pads that the red and black wires connect to (CONN1).

I have a Rancilio Silvia coffee machine, that needs time to warm up correctly in the morning – 30 minutes gets everything up to temperature, allowing me to draw excellent coffee shots.

For the past couple of years, I run a simple mechanical timer that turns on at 6am, and goes off at 8am which is fine for the days I go to work, but if I work from home, or it’s the weekend, I’m left with a cold coffee machine unless I turn it to manual mode – which often results in me forgetting to turn it off!

To get around this, I’d been planning on getting a smart switch which would integrate with my Home Assistant setup. I looked at the Belkin Wemo, but it seems a little expensive at $80 a pop.

I thought about designing and building my own one, but the prospect of messing around with mains power didn’t excite me, nor did 3D-printing a case – 3D printing plastic, by definition isn’t very fire resistant.

Browsing Hackaday.io, I stumbled across this project, and it got me wondering if I could find an ESP8266-based Australian WiFi outlet. After a bit of googling, I found something that at least looked the same for $AU20 delivered. Even if it wasn’t ESP8266, I could probably gut it and insert a custom board, so I ordered one.

It arrived pretty quickly.

I promptly pulled it apart. There was a daughter board that looked suspiciously like a ESP8266 although the footprint wasn’t the same as any reference design I could find.

So, I pulled the little metal shield off, and low-and-behold! An ESP8266! Now that I knew we were in business, I installed the iPhone application that comes with it, to try and work out how it was wired up.

The device

This particular switch has an obnoxious blue LED that is used to indicate power, and network connection (it flashes when not connected). It also has a red LED that is on when the relay is turned on. Finally, there is a button that allows you to turn the relay on and off manually.

Pins

A quick google, and I found the pin out of the ESP8266 chip – now it was just a matter of tracing the wiring back to each LED and the switch. I also needed to find where the TX, RX, Reset and GPIO0 were so I could program it.

Here is what I found:

GPIO 4 is the Blue LED

GPIO 5 is both the Red LED and the Relay

GPIO 13 is the button

I’ve marked the TX, RX, Reset and GPIO0 pins on the above image. By wiring these pins up to a FTDI cable, I was able to reprogram the switch, thus freeing me from the shackles of the crappy iOS app, and allowing my home-assistant.io server to talk to the switch.

Now, thanks to a Home Assistant and a NodeRed flow, the coffee machine will turn on between 6:30am and 1pm everyday UNLESS it detects that I’m not home. Nice!

Now that I have the proof of concept code running, it’s time to modify the built in Arduino core library that handles OTA updates.

The existing OTA library takes a binary object and an optional MD5 hash (to verify the upload), stores it in flash memory, then swaps the old binary out of the new binary and reboots the device.

To do verification via digital signatures, we need three additional pieces of information: the developers certificate (used to decrypt the hash), the encrypted hash, and the Certificate Authority certificate used to verify the developers signature.

The CA needs to be compiled in to the source code – there is little point sending that along with our payload.

The developer certificate and encrypted hash on the other hand, need to be supplied with the binary . One option is to upload the three files separately, but this would require extensive reworking of the updater API, and of the OTA libraries.

A better option would be to somehow bundle all three files in one package, which is the path I am looking to go down.

So, the first thing to do is work out what the file format looks like.

The binary blob is of an arbitrary size, and starts with the magic byte: 0xE9, which I assume is an instruction that is required to start the boot process.

Our certificate is also an arbitrary size. The signature will be fixed size, but dependent on the algorithm we use. Clearly we need some way of instructing the updater code where the boundaries for each file are.

We could pack them at the beginning, and set the byte before the file with the expected length – ie if our signature was four bytes, and certificate was 6 bytes it might look like this:

[ 4 | s | s | s | s | 6 | c | c | c | c | c | c | … ]

but that would mean we’d have to move the data around, as the bootloader would be looking for the magic number in the position 0. I’ve decided to do it the other way around – I’m going to use the last two bits to signify the lengths, and then count backwards. ie:

This produces a Bundle.bin file that can be uploaded.So far, I’ve managed to decode the lengths, and find where the two files I’m interested are live. Next I need to pull the files out, and do the verification. I think I’ll sign the binary using MD5 for the moment, as the updater class already has that function built in, so I effectively get it for free.

Comparing the SHA256 of a file after it has been uploaded allows us to check that it hasn’t changed. This doesn’t tell us if the file has been tampered with though – it would be easy enough for a someone to change the binary, and then change the hash so it matches.

To check the file was created by the person who said it was created by, we need to verify a cryptographic signature. The steps are fairly simple:

We upload the new binary, our public key and the signature file.

We check that the public key has been signed by a trusted certificate authority – if this fails, the CA can’t vouch for the person signing it, so we shouldn’t trust it.

We decrypt the signature file using the public key. This is the original SHA256 hash of the binary. If we can’t decrypt it, we can’t compare the hashes

We SHA256 the binary ourselves

We compare the hash we computed with the file that was uploaded. If the two hashes match, then the binary hasn’t been tampered with, and we can trust it.

The garage door opener has been running pretty well for the past couple of months, but I still have some work to do. I haven’t built out the configuration interface yet, and it turns out that if Home Assistant restarts, it forgets the last open state, so with out opening and closing the door again, I don’t know the state of the door.

This means I need to update the firmware.

The ESP8266 has facilities to do Over-the-Air (OTA) updates, however it doesn’t verify that the uploaded binary has been compiled by the person the device thinks it has. The easiest way to do this is to create a digest hash of the file and sign it. Then the device can verify the hash and check the signature matches.

There is an issue to implement this on the ESP8266 Github page, so I thought I would have a look at implementing something.

The first step is to be able to compare a hash. I decided to use the AxTLS library, as it has already been used for the SSL encryption on the device. After a google search, I found this page that outlines has to verify a SHA1 + RSA signature.

I simply pulled the sha1.c file (renamed it sha1.cpp), and created a sha1.h file that defines the functions in the cpp file. Next I created a test file, and hashed it using openssl:

So, I think I’ve worked out the meaning of the bitstream coming from the outdoor unit!

On my day off, I took the unit of the wall, got me some coffee and setup shop in the hallway, oscilloscope in hand.

I must admit, I’m still getting used to using the oscilloscope and I’m sure there is a far better way to do what I’m trying to do, but I found that if I probe the RX pin on the CPU, with the ‘scope set to single trigger mode and keep hitting the start button, I’d eventually align the waveform at the start of the cycle. After that I used the onscreen rulers to work out the gaps between the pulses. I then wrote them down in to this spreadsheet. I’d change a setting, take a new set of readings, and repeat until I had covered enough states that I could get a complete picture of what was going on.

Looking at the data, I could start to see some patterns.

The shortest spacing was around 2ms (some longer; some shorter)

The RX pin is idle low, and there is always a high transition to represent the start bit

There seems to be a low transition to represent a stop bit

There is 9 bits between the start and stop bit (except for the last set)

It’s starting to look like a straight up serial transmission, except the idle state, start and stop bits are inverted, so unfortunately the built in serial protocol decoder wouldn’t read it.

Next I need to find the bits that change between each state.

The power bit was pretty obvious: there was only one bit that was different when the power was off – the 68th bit.

Looking at the rest of that byte, there was a pattern developing in the next 3 bits – they seemed to change when the settings changed. Taking LSB first, Fan only mode is represented by 0x01, Humidity mode (Yeah – I don’t know what that is either) is 0x02, Cool mode is 0x03, Heat mode is 0x04 and Auto mode is 0x05. The next three bits represent the fan speed: Auto0x00, Speed 2: 0x02, Speed 3: 0x03, Speed 4: 0x04. But was was the ninth bit?

Having a think about serial, it’s could a parity bit. By summing the number of bits, it became pretty obvious it was odd parity. I checked this against the other bytes, and it checked out – now we are getting somewhere!

Looking at the next byte, it was clear it was changing with the temperature. I purposely looked at the lowest possible setting for the temperature (15deg) and the highest (30deg) and it was here I was lead down the garden path a little. Reading up on other people’s efforts at reverse engineering air conditioner units, this is a fairly common range. Many of the IR transmitters represent this as a 4 bit number, where 0x0 is 15 and 0xF 30. Unfortunately, I couldn’t for the life of me work out how that mapped to the numbers I was seeing.

It turns out, this system uses a 5 bit number – feasibly being able to represent 0 – 31 degrees. Bits 6 and 7 are always 0, and bit 8 is the “economy” settings.

There is four unknown bytes, and one block that seems to be make up of 5 bytes. My guess is one of the unknown bytes is reserved for errors, and one is a serial number of some sort. I have no clue what the other two could be for, and I’m quite confused by the last, short byte.

But this is definitely progress!

I did a final check the get some timing on what is transmitted, and there seems to be three windows of roughly 212ms each. The first from the outdoor unit, the second transmitted by the remote control, and I’m guessing the third is for a slave unit.

To build a test harness, I’ll need to bit-bang the data for 212ms, then set the line to high impedance for 424ms. This will hopefully allow me to get the remote control to work on my bench. Once I can get the remote to work, I can analyse what it is doing. Next, I’ll simulate that as well, then set the remote to slave mode and work out that part of the protocol. Once I have the three parts of the protocol nutted out, I can just simulate the outdoor unit, connect the spare remote controller as master, and the microcontroller will become the slave. Easy!

I hacked together a little node-red script that listens for events from my homeassistant.io installation using a quick eventsource module that I knocked up, which is generally working pretty well – occasionally, it selects the wrong input, because I’m relying on my CEC hack but I’ll deal with that later.

Time to knock up a quick case.

I did up a quick design in FreeCAD and printed it out. Originally, I based the design off the Apple TV, as I thought I could have some sort of visual symmetry.

It looked terrible.

It was way bigger than it needed to be, and looked cheap and nasty. And once I decided to add CEC, I needed access to the HDMI port, so a redesign was in order.

I came up with a second design (top, bottom, foot), which hugged the contours of the PI. I also dropped the cutout for the IR plexglass, instead making a feature of the LEDs and IR receiver.

After printing it out, I put everything together and placed it next to my receiver. But there was a problem – the network kept dropping out. It turns out that as the temperature rose inside the case, the wifi chip would reset. Boo.

I tried drilling some holes in it, and it didn’t make too much difference. At this point, it was Christmas, so I removed the top of the case and hide it behind the receiver (I didn’t need the IR bit at the moment – the CEC did what I needed).

Fast forward a month, and I decided to revisit. I had just bought a Raspberry PI 3, with an official case for a project at work, and I noticed that it had no air holes, so I wondered if something was wrong with the WIFI dongle. It was sitting at a weird angle over a chip that did get warm, so I unsoldered it, and re-soldered it at a different weird angle, away from the chip.

Of course, I overcooked it.

Luckily, I had a spare, which looks like a more robust unit. I took that one apart, and soldered it in.

I’m sure it’s breaking some sort of USB spec, but it works.

I put it back in the case

and screwed it back together

The case still isn’t perfect. The front left corner needs a screw stalk (The gap is because there is nothing holding it together). I could fix it with some sort of clip, but I’m thinking about a completely different design, which will have LEDs on an angle and on the back, but that would require a new PCB, so I’ll stick with this for the moment. I still can’t work out how to get a less streaky top. I clearly need some more 3D printing practice.

Here is a picture of it in situ:

It’s on an angle, because I bounce the IR off the coffee table. The audio receiver works perfectly, the foxtel works pretty well (though that is more to do with my LIRC setup). The TV doesn’t really work – it’s a bit far away, which is why I want to redesign the case.

It’s good enough for now. I’m going to spend some time on the software.

I took the remote unit off the wall again, and this time removed the signal wire fro the remote and attached it to my Oscilloscope.

And this is signal that comes from the outdoor unit.

I’m not sure if I stuffed up my reading the last time, but it looks like the pulse width is 2ms.

Really, I needed to replay this and see if I could get my test unit to initialise. I thought about using an Ardiuno, so I googled bit banging serial to see the best way to do it. One of the results that caught my eye was another Hackaday article entitled “Introduction to FTDI bitbang mode“. I had literally just cleaned up my workbench and found a FTDI module. Perfect!

I knocked up a little circuit that drove a transistor from 0V to 12V, and adapted the code from the article to control the FTDI modules CTS line. I had to reduce the sleep time to 1.8ms to adjust for kernel context switches (I’m guessing) while talking to the adapter. I got it pretty close to 2ms though.

I wired it up to the controller, and got one step closer – now instead of timing out and flashing C0 12, it just sits flashing “9C” forever.

My guess? This communication protocol works on one-wire – I’m not releasing the line, so the remote never gets a chance to send a response. It looks like I’ll need some sort of tri-state buffer, so I can set the line to high-impedance after I’ve sent the preamble.

I was curious to see if I could get any other clues to the protocol, so I started poking around the big chip on the PCB. One of the pins receives the same signal the signal line does, except it’s inveted and 0-5V! I went and looked up the chip (it’s a UPD78F0393 from NEC – I’m so glad the remote manufacture labelled all their chips nicely), and that pin (#75) is labelled RXD0. That sounds like a serial receive line to me!

Pin 76 is labelled TXD0, which I’m guessing is the transmit line. This should make decoding stuff way easier, because I’ll be able to see what is actually being transmitted and received separately. Win!

I’m going to try and trace out the front-end to this – so far I see a NJM2904 (an op-amp) is on the path – my guess is that is the thing inverting the signal and driving it to 12V. Tracing this circuit out should allow me to build a compatible circuit from my microcontroller.

I get it on my bench, and test out my theory – if I’m right, it should boot up and start sending commands when the buttons are pressed.

I was wrong.

The unit just sits there flashing “9C” for a couple of minutes, then failing over to a “C0 12” error. The Oscilloscope was no use either – I just saw a constant 12V on the signal wire.

Hmmm.

Looks like I’ll have to pull off the real wall unit.

Using some wire and alligator clips, I extended the wires so I could reach them with the scope.

This time I got somewhere – I could see a signal!

The pulse width is around 1.04ms, going from 12V to 0V. Weird.

I go distracted for a while trying to decode the protocol – is there start bits and stop bits? What about a parity bit?

I knocked together a quick D3 script (I’m a web developer, remember – I use web technology for a lot of this stuff because that is what I’m used to) to display the wave form. First, I wrote a ruby script that created a CSV file of just the transitions. There are two entries for each transition – a 0V and 12V value – so the graph ends up looking like a binary stream.

I then wrote another script that aligned the stream so each pulse was exactly 1.04, and each pulse hight was 12V or 0v. Finally, I scaled everything so the pulse width was 1, as this made reading the graphs easier.

I ended up with some pretty graphs like this:

There was still a problem though – I didn’t have a baseline for the communications.

I knew that the control unit didn’t send any data unless it was connected to the outside unit. I also knew that changing the temperature changed some of the bits in the data stream, so clearly there was some half-duplex serial communication going on. I needed to find out what the outdoor unit sent to initialise the control unit…

The first issue: I had no idea what the model number was – it’s not written on the unit, nor on the instruction manual. So I just googled for Fujitsu airconditioner remote, hit image results and looked for one that looked the same. Once I found it and clicked through to the source page, I found out that it is a UTB-YUB/GUB/TUB (There are three model numbers depending on where in the world you are).

I found a supplier on ebay (who was actually Melbourne based), who had a new remote unit for $60, which I bought as I wanted a test unit on my workbench – mainly because trying to test things using the unit on the wall would be really annoying.

While I waited for it to arrive, I continued googling to find as much info as I could about it. Thankfully, a number of airconditioning repair places have their installation manuals online. Reading though the them, it was clear there was a three wires that connect the remote to the outdoor unit – +12V, GND and a signal.

Bingo.

Now, I need to work out what this signal wire does.

My first hypothesis was the remote unit worked a lot like an IR remote – every button press sent the complete state to the outdoor unit. If this was the case, it should just be a matter of hooking up a DSO (I have the LabNation SmartScope), copying the signal and replaying it via a microcontroller.

While I could pull the one off the wall, I patiently waited for my test unit to arrive.

I am not the first person to build an IR blaster for a RaspberryPI, and I sure won’t be the last. Thanks to the LIRC gpio module, the circuit required is super simple:

One side is the transmitter – 3 IR LEDs in series, with a 56ohm resistor, driven by a bog-standard BC547 transistor.

The LEDs I used have a 1.2V at 20mA. forward voltage, so the three in series drop 3.6V. R2 needs to drop 1.4V (to add up to 5V). R = V/I, so R2 needs 70 ohm. For some reason, I picked a 56 ohm resistors, so the LEDs will get driven a little harder at 25mA, which is still well with in their spec (They max out at 50mA).

The transmitter side is even easier – the device does all the work, so there is just a pull down resistor on the signal leg. I picked GPIO 17 and 18 at random – any GPIO line will work, and you can configure it in software.